In the prior art, a hydraulic operating unit (hydraulic power servo mechanism) of a friction engaging apparatus (such as a multi-plate clutch, multi-plate brake and pedal brake) used in a transmission for an automobile or the like is composed of a cylinder, a single piston and a return spring, etc. There has been much to be desired in the art for making the hydraulic operating unit hereinabove mentioned as compact as possible subject to maintaining requisite quick and stable actuation and release. Particularly, a larger diameter of the piston has been required for the single piston corresponding to a low hydraulic pressure in the case where different hydraulic pressures are applied to actuation and release.